The present invention relates to a thermal fatigue prevention apparatus for a high temperature pump such as a reactor coolant recirculation pump for treating a high temperature pumping liquid, and more particularly to a thermal fatigue prevention apparatus in which a low temperature seal purging liquid is supplied into a shaft sealing chamber for cooling and cleaning a shaft sealing mechanism, and in which a part of the seal purging liquid is caused to flow into a high temperature pump casing through a pump shaft through hole.
In the region of a shaft through hole 12 formed between a pump casing 1 and a pump shaft 4 of a pumping system as shown in FIG. 12 by way of example, there is commonly provided a shaft sealing mechanism such as a mechanical seal 14. Through the shaft through hole 12, a seal purge liquid A having a suitable temperature (usually of the order of ordinary temperature) is led into a shaft sealing chamber 6a from the outside for the purpose of cooling and cleaning the shaft sealing mechanism 14.
In this case, when a pumping liquid B is of a high temperature, there arises a great temperature difference between the inside of the shaft sealing chamber 6a which is kept at a low temperature due to the seal purging liquid A, and the inside of the casing 1 through which the pumping liquid B circulates. Such a temperature interface appears in a region C where the low temperature seal purging liquid A flowing into the pump casing 1 from the shaft sealing chamber 6a through a casing cover 2 is mixed with the high temperature pumping liquid, which causes a dramatic temperature fluctuation phenomenon during the irreversible mixing process of a low temperature liquid and a high temperature liquid. As a result, a fluctuating thermal stress is generated within the adjacent metallic structure, which may bring about cracks in the metal.
Conventionally to cope with such problem, sleeves 15a and 15b, which can be replaced with new ones whenever thermal fatigue occurs, are arranged on the metallic part in the mixture region C of liquids A and B as shown in FIG. 13. However, this replacement work is troublesome because a main bolt 13 must be removed (FIG. 15). Also, in order to check for the occurrence of the thermal fatigue, there is a need for a disassembling check which is extremely troublesome in the reactor coolant recirculation pump because of the possible presence of radiation.
Furthermore, Japanese Examined Patent Publication (Kokoku) No. 64-4160 discloses a thermal barrier 16 arranged within a journal 11 (FIG. 15) of the submerged bearing 10 as shown in FIG. 14, thereby raising the temperature of the seal purging liquid A to consequently reduce the temperature difference between the seal purging liquid A and the pumping liquid B. However, in this known technique it is substantially impossible to obtain an adequate temperature rise effect by means of a thermal barrier 16, which leads to a difficulty in preventing thermal fatigue.
FIG. 15 shows a reactor coolant recirculation pump, that is, a representative pump treating a high temperature water, to which the present invention is applied. In the drawing, within a pump casing 1, an impeller 3 is rotatably supported by a journal 11 of a submerged bearing 10. A pump shaft 4 of the impeller 3 projects from a shaft through hole 12 of a casing cover 2, and is drivingly connected to a motor (not shown) through a coupling 7 within a motor pedestal 8 mounted on the casing 1 by means of main bolts 13. Between the pump shaft 4 and the casing cover 2, there is provided a shaft sealing device 6 including a mechanical seal 14. This shaft sealing device 6 is provided with a shaft sealing chamber cooler 5 which is in turn equipped with a seal purging liquid supply inlet 5a. Besides, reference numeral 9 in the drawing denotes an impeller for circulating the seal purging liquid.
In this pump, a low temperature (e.g., about 40.degree. C.) seal purging liquid A of, for example, 5 liter/minute is supplied into a shaft sealing chamber 6a (FIG. 12) of the shaft sealing device 6. Among the liquid A, about 3 liter/minute, for example, of seal purging liquid is discharged through a mechanical seal 14 to the outside of the pump. Therefore, a remainder of about 2 liter/minute is permitted to flow into the pump casing 1 through an annular gap of the shaft through hole 12 formed between the casing cover 2 and the pump shaft 4. As the temperature of the pumping liquid B inside the pump casing 1 is usually about 280.degree. C., the temperature difference .DELTA.T becomes 280-40=240(.degree.C.). Thus, a temperature fluctuation corresponding to temperature difference .DELTA.T is generated in the vicinity of the outlet of the shaft through hole 12. The maximum thermal stress .DELTA..sigma. produced on the metal surface due to the temperature fluctuation is expressed as: EQU .DELTA..sigma.=E.beta..DELTA.T.eta./(1-.nu.)
where
temperature fluctuation on metallic material surface/water temperature fluctuation&lt;1 PA1 E: Elastic modulus of material PA1 .beta.: coefficient of linear expansion of material PA1 .nu.: Poisson's ratio of material
When the material is made of austenitic stainless steel, an allowable fluctuation stress based on a fatigue limit of the material is .DELTA..sigma./2.ltoreq.9 kg/mm.sup.2 (one sided amplitude), and hence it is estimated that the allowable water temperature fluctuation amount .DELTA.Tmax is 100.degree. C. at most. In other words, previous to mixing with the high temperature pumping liquid B, the temperature of the low temperature seal purging liquid A must be raised so as to present a temperature difference of 100.degree. C. or less. In the case of the above pump, there is required a heater capable of raising the temperature of about 2 liter/minute of the seal purging liquid A of 40.degree. C., up to 180.degree. C. (=280-100) or more. The above-mentioned structure employing the thermal barrier 16 and the like is insufficient, and may require an additional heater.
It is an object of the present invention to provide a thermal fatigue prevention apparatus for a high temperature pump having a simple structure and high reliability and being capable of raising the temperature of the seal purging liquid up to the appropriate temperature to prevent thermal fatigue.